Generally, fine pattern formation is carried out by photolithography in the manufacture of a semiconductor device. A number of substrates called photomasks (transfer masks) are normally used for such fine pattern formation. The photomask comprises generally a light-transmissive glass substrate having thereon a light-shielding fine pattern in the form of a metal thin film or the like and the photolithography is used also in the manufacture of the photomask.
In the manufacture of a photomask by photolithography, use is made of a mask blank having a light-shielding film on a light-transmissive substrate such as a glass substrate. The manufacture of the photomask using the mask blank comprises a writing process of writing a required pattern on a resist film formed on the mask blank, a developing process of developing the resist film to form a resist pattern in accordance with the written pattern, an etching process of etching the light-shielding film along the resist pattern, and a process of stripping and removing the remaining resist pattern. In the developing process, a developer is supplied after writing the required pattern on the resist film formed on the mask blank to dissolve a portion of the resist film soluble in the developer, thereby forming the resist pattern. In the etching process, using the resist pattern as a mask, an exposed portion of the light-shielding film, where the resist pattern is not formed, is dissolved by dry etching or wet etching, thereby forming a required mask pattern (light-shielding film pattern) on the light-transmissive substrate. In this manner, the photomask is produced.
In order to miniaturize a pattern of a semiconductor device, it is necessary to shorten the wavelength of exposure light for use in photolithography in addition to miniaturization of the mask pattern of the photomask. In recent years, the wavelength of exposure light for use in the manufacture of a semiconductor device has been shortened from KrF excimer laser light (wavelength: 248 nm) to ArF excimer laser light (wavelength: 193 nm).
On the other hand, with respect to the photomask and the mask blank, the miniaturization of the mask pattern of the photomask requires a reduction in thickness of the resist film formed on the mask blank and dry etching as a patterning technique in the manufacture of the photomask.
However, the reduction in thickness of the resist film and the dry etching have the following technical problems.
One problem is that the processing time of the light-shielding film exists as one serious restriction to the reduction in thickness of the resist film on the mask blank. Chromium is generally used as a material of the light-shielding film and, in dry etching of chromium, a mixed gas of chlorine gas and oxygen gas is used as an etching gas. When patterning the light-shielding film by dry etching using the resist pattern as a mask, since the resist film is an organic film mainly composed of carbon, it is very weak against an oxygen plasma forming a dry etching environment. While patterning the light-shielding film by dry etching, the resist pattern formed on the light-shielding film should remain with a sufficient thickness. As one index, in order to make excellent the sectional shape of the mask pattern, the resist film is required to have a thickness that still remains even when the etching time is about twice a just etching time (100% overetching). For example, since, in general, the etching selectivity of chromium as the material of the light-shielding film to the resist film is 1 or less, the thickness of the resist film is required to be twice or more that of the light-shielding film. Although, naturally, the reduction in thickness of the resist film is desirable for achieving improvement in pattern CD accuracy, there is a limitation thereto for the reason described above.
Following the miniaturization of circuit patterns in recent years, however, a requirement to the photomask CD accuracy is becoming more strict. As the integration of semiconductor integrated circuits increases, the design rule in the manufacture of semiconductor integrated circuits becomes stricter and, in particular, a requirement to the photomask CD uniformity and CD linearity is becoming much stricter. Further, following the miniaturization of circuit patterns, there arises a problem that diffraction of exposure light deforms a transfer pattern to degrade pattern resolution and, as a solving means therefor, use is often made of the optical proximity correction (OPC) technique that predicts deformation of a transfer pattern by simulation in advance and corrects it. According to this OPC technique, the deformation of the transfer pattern is suppressed by providing the transfer pattern with a finer auxiliary pattern and therefore a resist pattern becomes still finer. Consequently, there arises a problem that the ratio of a resist film thickness to a pattern line width of the resist pattern (aspect ratio) increases so that there occur pattern collapse or stripping and lack of space pattern resolution.
In view of this, Japanese Unexamined Patent Application Publication JP-A-2005-62884 (Patent Document 1) discloses a method of using a hard mask pattern instead of a resist pattern in dry etching of a chromium light-shielding layer in order to improve the photomask CD uniformity and to reduce the thickness of a resist film. The technique disclosed in Patent Document 1 uses a hard mask layer made of a conductive substance having an advantage of the hard mask pattern and further advantages that, since it is conductive, electron beam writing can be performed well and that a large etching selectivity can be assured with respect to the chromium light-shielding layer. As the substance forming the hard mask layer, there are enumerated Mo, MoSi, MoSiON, Hf, Hf compounds, Zr, Sn, Fe, NiSi, CoSi, compounds thereof, and so on.